The transformation of a .DELTA..sup.5 -steroid to a 7-keto-.DELTA..sup.5 -steroid has long been known in the steroid literature as a desirable means for the preparation of intermediates which are then converted to many other pharmaceutically useful steroids. For example, cholesteryl acetate is transformed to 7-ketocholesteryl acetate. See Dauben, et al. J. Org. Chem. 34, 3587 (1969). The intermediate 7-ketocholesteryl is then converted to cholesta-5,7-diene-3-ol 3-acetate (provitamin D.sub.3) see Dauben, et al. J. Org. Chem. 36, 3277 (1971). Provitamin D.sub.3 is then converted to Vitamin D.sub.3 by methods well known to those skilled in the art.
Similarly 17.alpha.-methyl-androsta-5-ene-7-one-3.beta.,17.beta.-diol diacetate is formed by the oxidation of 17.alpha.-methyl-androsta-5-ene-3.beta.,17.beta.-diol diacetate. 17.alpha.-Methyl-androsta-5-ene-7-one-3.beta.,17.beta.-diol diacetate is then converted to calusterone (7.beta.,17.alpha.-dimethyltestosterone) a useful anabolic agent and gonadatropin inhibitor. See U.S. Pat. No. 3,654,320.
The known methods for oxidizing .DELTA..sup.5 -steroids to 7-keto-.DELTA..sup.5 -steroids include sodium chromate in conjunction with acetic anhydride and acetic acid but this demands long reaction time and undesirable by-products are formed. t-Butyl chromate has also been used but this is a hazardous reagent and is best avoided. Bispyridine chromium trioxide is known to achieve the oxidation cleanly, but again reaction times are long and large excesses of chromium reagent are required. In short the oxidation methods available are either hazardous, time consuming or inconvenient in their use.
A chromiun trioxide-pyrazole complex has been described as a reagent for the oxidation of alcohols. See Corey and Fleet, Tetrahedyron Letters 4499 (1973).
The surprising discovery has been made that a chromium trioxide-pyrazole complex serves as an efficient oxidizing agent for the allylic oxidation of a .DELTA..sup.5 -steroid. This oxidation is accomplished best at low temperatures in a few hours. The work-up is simple and the pyrazole is readily recovered.